1. Field of the Invention
This invention relates to the field of operating multiple magnetic electronic article surveillance (EAS) systems, and in particular to wireless synchronization of such multiple EAS systems without wires, cables, fiber optic links and the like between individual ones of the multiple EAS systems.
2. Description of Related Art
Pulsed magnetic EAS systems, for example, operate by generating a short burst of magnetic flux in the vicinity of a transmitter antenna. This pulsed field stimulates a particular type of magnetic label or marker, whose characteristics are such that it is resonant at the operating frequency of the system. The marker absorbs energy from the field and begins to vibrate at the transmitter frequency. This is known as the marker's forced response. When the transmitter stops abruptly, the marker continues to ring down at a frequency which is at, or very near the system's operating frequency. This ring down frequency is known as the marker's natural frequency. The vicinity of the transmitter antenna in which the response can be forced is the interrogation zone of the EAS system.
The magnetic marker is constructed such that when the marker rings down, the marker produces a weak magnetic field, alternating at the marker's natural frequency. The EAS system's receiver antenna, which may be located either within its own enclosure or within the same enclosure as the transmitter antenna, receives the marker's ring down signal. The EAS system processes the marker's unique signature to distinguish the marker from other electromagnetic sources and/or noise which may also be present in the interrogation zone. A validation process must therefore be initiated and completed before an alarm sequence can be reliably generated to indicate the marker's presence within the interrogation zone.
The validation process is time-critical. The transmitter and receiver gating must occur in sequence and at predictable times. Typically, the gating sequence starts with the transmitter burst starting with a synchronizing source, such as the local power line's zero crossing. The receiver window opens at some predetermined time after the same zero crossing. Problems arise when the transmitter and receiver are not connected to the same power source. In a three phase power system, power lines within a building can have individual zero crossings at 0.degree., 120.degree. or 240.degree. with respect to each other.
Some noise sources are synchronous with the local power line. Televisions, monitors, cathode ray tube in other devices, electric motors, motor controllers and lamp dimmers, for example, all generate various forms of line synchronous noise. As a result, no one time window can be guaranteed to be suitable for detecting markers. Accordingly, pulsed magnetic EAS receivers typically examine three time windows to scan for the presence of magnetic markers, as illustrated in FIG. 4. With a 60 Hz power line frequency, for example, the first window occurs nominally 2 milliseconds (msec) after the receiver's local positive zero crossing; by convention, referred to as phase A. The second receiver window, referred to as phase B, occurs 7.55 msec after the local zero crossing; being determined by adding one-third of the line frequency period and 2 msec. The third receiver window, referred to as phase C, occurs 13.1 msec after the local zero crossing; being determined by adding two-thirds of the line frequency period and 2 msec. At 50 Hz power line frequencies, the timing is analogous. Each receiver window begins a nominal 2 msec after either the 0.degree., 120.degree. or 240.degree. point in the line frequency's period. In this way, even if a first EAS system, referred to as system A, is connected to a different phase of the power line than a nearby EAS system, referred to as system B, the transmitted signal of system B will not directly interfere with the receiver of system A.
In order to compare received signals to background noise, separate noise averages are continuously sampled, computed and stored as part of a signal processing algorithm. This is commonly done by operating the EAS systems at 1.5 times the power line frequency, 90 Hz for a 60 Hz line frequency or 75 Hz for a 50 Hz line frequency, and alternating the interpretation of each successive phase. More particularly, if phase A is a transmit phase (the receiver window is preceded by a transmitter burst), phase B will be a noise check phase (the receiver window was not preceded by a transmitter burst), phase C will be a transmit phase, phase A will be a noise check phase, and so on.
Even if the EAS systems synchronize to their respective zero crossings, independent pulsed magnetic EAS systems operating adjacent or in close proximity to each other can have a degrading influence on each other. Assume, for example, a situation wherein two independent EAS systems are installed in close proximity to each other, but connected to different legs of the power line. One system transmits in phase A and the other system transmits in phase B, with respect to the first system. If a valid marker is located between the antennas of these two independent systems, the phase A system will sense the ring down response in the phase A receiver window. In phase B, the second system transmits and stimulates the marker into another ring down response. The first system did not transmit and is expecting a lower level noise response in its phase B window. Instead the first system detects the ring down response from the marker, without having previously transmitted, and exits its validation sequence, deciding on the basis of its programming that the detected signals must have been noise. Likewise, the second system detects the marker in the window following phase B and enters a validation sequence. In phase C, when the second system expects the marker signal to be absent, the marker is stimulated by the first system, which is again transmitting in phase C. The second system senses the ring down signal in its phase C window, when it did not transmit, decides the detected signal must have been noise in accordance with the programming, and exits its validation sequence. Thus, two systems in close proximity which are not phase synchronized can inhibit each other. The phrase close proximity is used herein as for denoting when two or more EAS systems, for example pulsed EAS systems, are close enough to interfere with one another if not synchronized in one fashion or another.
Previous implementations of pulsed magnetic EAS systems, for example those available from Sensormatic Corporation, have utilized two approaches to synchronization. One approach is manual, fixed phase operation at the power line frequency. According to this approach, a system installer determines the quietest phase and sets the system to expect marker signals only in that phase. This can be effective, but relies on the assumption that the quietest phase will always remain the quietest phase. In fact, many noise sources are not so constant and the system's performance can vary throughout the day and from day to day. A second approach is hard wired operation, either at the power line frequency or at 1.5 times the line frequency, wherein all EAS systems operating in close proximity are wired together. One EAS system is designated the master and a synchronizing signal is sent over wires, cables or optical fibers to ensure that subordinate or slave EAS systems all operate in phase with the master. This method is also effective, but requires connection of some form of control cable between respective system processor boards of the multiple EAS systems. Such connections can be inconvenient and can add significant cost if, for example, the installation requires routing the cable under the floor.
Pulsed EAS systems can incorporate special features, such as frequency-hopping or operating at two slightly different frequencies to improve detection of markers with a broader manufacturing tolerance for center frequency. Phase flipping, wherein the two coils which constitute the system's transmitter antenna alternately reverse their phase relationship between aiding (0.degree.; also referred to as in-phase) and figure-8 (180.degree.; also referred to as substantially out-of-phase) operation. This technique improves overall detection of magnetic markers throughout the system's interrogation field, since locations and marker orientations which would cause signal nulls when the transmitter coils were in the aiding mode, for instance, will be absent in the figure-8 mode and vice versa.
If an EAS system is operating at 1.5 times the line frequency, for example, it is not automatically known which line phase to operate in when the system is first powered up and completes its self-test routines. It is important to have adjacent transmitters operating in the same phase, that is A, B or C, for two reasons. The first reason is that the transmitter fields can aid each other, improving stimulation of magnetic markers within the interrogation zones of both systems. The second reason is that if two adjacent EAS systems are operating out-of-step and a marker initiates a validation sequence in a first EAS system, a second, adjacent EAS system will stimulate the marker in what would be one of the first system's noise windows, which would force the first EAS system out of the validation sequence, reducing overall performance.